The invention relates to a mobile work machine having a chassis (10) and with two front and two rear support brackets (20, 22). The support brackets can be displaced from a driving position close to the chassis into a support position by changing the base angles (α) thereof and/or can be telescoped by changing the lengths thereof between the end on the mounting side and that on the free end. In order to be able to automatically determine the support leg positions (Xc/Yc) in relation to the chassis (10), three transceiver units (S1/E1, S2/E2, S3/E3) related to the support legs are provided for transmitting and receiving run time or distance signals. Furthermore, a microprocessor-supported evaluation unit responding to the transmitted and received signals of the transceiver units arranged in pairs is provided, comprising a software routine for determining the support leg positions in a coordinate system (x/y) fixed to the chassis.